a) Field of the Invention
The present invention relates to polycrystalline semiconductor material suitable for acquiring a large carrier mobility, its manufacture method, semiconductor devices using such material, and its evaluation method.
Amorphous silicon has been used as a material of a channel layer of a thin film transistor (TFT) used for active matrix type liquid crystal display panels. If amorphous silicon is replaced by polysilicon, carrier mobility in the channel can be made high. As the carrier mobility becomes higher, the maximum drive current of a transistor becomes large. It is therefore possible to reduce the area occupied by TFT in each pixel area of a liquid crystal display panel and make the aperture ratio high. It is also possible to increase the number of pixels per unit area of a substrate and to realize a high precision liquid crystal display panel.
b) Description of the Related Art
A polysilicon layer contains grain boundaries and grain defects which form localized states (localized level). These localized states lower carrier mobility or increase leakage current so that the performance of a semiconductor device is degraded. It is known that if hydrogen atoms are doped in a polysilicon layer, grain boundaries and grain defects are made electrically inactive and the performance of the semiconductor device is improved (JP-A-62-84562).
Known methods of doping hydrogen atoms into a polysilicon layer include exposing the polysilicon layer to hydrogen plasma (JP-A-63-46775), forming an SiO2 film or SiN film containing hydrogen atoms on the polysilicon layer and heating it (JP-A-6-314697, JP-A-8-32077), and heating the polysilicon layer in hydrogen gas (JP-A-63-200571).
Although these conventional methods can increase carrier mobility in a polycrystalline semiconductor layer to some degree, technologies of forming a polycrystalline semiconductor layer having a much larger carrier mobility have been long desired.
It is an object of the present invention to provide polycrystalline semiconductor material capable of increasing carrier mobility, its manufacture method, semiconductor devices using such material, and its evaluation method.
According to one aspect of the present invention, there is provided a polycrystalline semiconductor material essentially comprising Si, Ge or SiGe, wherein the material contains H atoms and the number of monohydride structures of couplings between Si or Ge, and H is larger than the number of higher-order hydride structures.
According to another aspect of the present invention, there is provided a polycrystalline semiconductor material essentially comprising Si, Ge or SiGe, wherein the material contains H atoms and a peak intensity of a monohydride structure in a local vibrator mode measured by a Raman spectral analysis is higher than a peak intensity of a higher-order hydride structure.
According to another aspect of the present invention, there is provided a method of forming a polycrystalline semiconductor layer, comprising the steps of: forming a polycrystalline semiconductor layer essentially comprising Si, Ge or SiGe, on a support substrate; and adding hydrogen to the polycrystalline semiconductor layer so that the number of monohydride structures of couplings between Si or Ge, and H is larger than the number of higher-order hydride structures.
According to another aspect of the present invention, there is provided a method of forming a polycrystalline semiconductor layer, comprising the steps of: forming a polycrystalline semiconductor layer essentially comprising Si, Ge or SiGe, on a support substrate; adding hydrogen to the polycrystalline semiconductor layer; and dissociating hydrogen in the polycrystalline semiconductor layer added with hydrogen by heating the polycrystalline semiconductor layer so that the number of monohydride structures of couplings between Si or Ge, and H is larger than the number of higher-order hydride structures.
With the structures described above, a polycrystalline semiconductor layer having a high carrier mobility can be formed.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a support substrate having an insulating surface; a polycrystalline semiconductor layer formed on the insulating surface of the support substrate, the polycrystalline semiconductor material essentially comprising Si, Ge or SiGe, wherein the material contains H atoms and the number of monohydride structures of couplings between Si or Ge, and H is larger than the number of higher-order hydride structures; a gate insulating film formed on a local surface area of the polycrystalline semiconductor layer; a gate electrode formed on the gate insulating film; and a source electrode and a drain electrode in ohmic contact with the polycrystalline semiconductor layer on both sides of the gate insulating film respectively.
With this structure, a TFT having a high electric field mobility can be formed.
According to another aspect of the present invention, there is provided a method of evaluating a semiconductor device comprising the steps of: irradiating a laser beam to a channel region of a thin film transistor formed on the surface of a transparent substrate, the channel region essentially comprising Si, Ge or SiGe; and comparing a peak intensity of a monohydride structure of couplings between Si or Ge, and H with a peak intensity of a higher-order hydride structure, through observation of spectra of light scattered from the channel region.
By comparing a peak intensity of the monohydride structure with a peak intensity of the higher-order hydride structure, carrier mobility in the channel region can be estimated.
As described above, carrier mobility can be made high by controlling the addition of hydrogen to a polycrystalline semiconductor material. By forming a TFT using such polycrystalline semiconductor material, TFT having a high electric field mobility can be realized with improved off-current characteristics, threshold value and the like.